Irish show jumper Cian O'Connor's Olympic gold medal is in jeopardy after a sample taken from his horse was found to contain a banned substance.

The Equestrian Federation of Ireland said on Friday that it had been informed of Waterford Crystal's positive test at the Athens Games.

A statement from O'Connor said he was "devastated by the news".

"I wish to assure everyone that I have competed honestly and honourably both for myself and my country," he said.

"I absolutely believe that no performance-enhancing drugs were given to the horse," O'Connor added.

However, the Irish federation statement said that the horse's vet James Sheeran had acknowledged administering a sedative to the horse as part of treatment for an injury, a month before the Games.

"Mr O'Connor was told by his vet that the sedative drug would disperse from the horse's system within 10 to 14 days, and was not in any case a factor that would influence performance."

I am very professional, and take everything extremely seriously, both veterinary matters and the stable management

Cian O'Connor

O'Connor has said that he will make a detailed case to the Federation Equestre Internationale (FEI) when the results of the 'B' sample become known.

"I am fully aware of the FEI rules and I know that I must take responsibility for any medication
administered to my horse," added O'Connor.

"I am very professional, and take everything
extremely seriously, both veterinary matters and the stable management at this level of competition."

Equestrian Federation of Ireland President Avril Doyle said that while the body did "not condone under any circumstances the use of prohibited
substances...the federation stands behind Cian O'Connor in his bid to clarify the situation."

Under international rules, if the B-sample is also positive, the person responsible must provide an explanation for the horse's positive test
within a further 10 days.

Subsequently, the international federation's judicial
committee will announce a decision on the case.

The person responsible then has 30 days to appeal any decision to the Court of Arbitration for Sport.

The international body said 40 horses - 20 percent of those participating in Athens - had been tested.

O'Connor became an Irish national hero when he claimed the country's only medal in Athens.

It was further revealed on Friday that the horses of German riders Ludger Beerbaum and Bettina
Hoy also failed tests during the Games.

Beerbaum's horse Goldfever tested positive for Betamethasone after the German helped his country win the team show jumping title while Hoy's horse Ringwood Cockatoo failed a test for hydroxy-diphenhydramine.

And the horse of three-day eventer Harald Riedl, Foxy, also failed a drugs test at the Olympics, according to the president of the
Austrian equestrian federation.